Page 99 of Rising Seas: Past Present, and Future features the astronomical theory of the ice ages proposed by Milutin Milankovitch (1879-1958), a Serbian mathematician and geophysicist. Ice ages begin with a low Earth axial tilt angle (obliquity), a highly elongated orbit around the Sun (eccentricity), and greatest Earth-Sun distance on June 21 (Northern Hemisphere summer solstice; precession), and vice-versa. The theory was widely accepted, after scientists in the 1970s discovered 100,000, 42,000, and 23,000 year oscillations in oxygen isotopes ratios of tiny marine organisms, which correspond to the orbital cycles. What connection (if any) exists between ice ages and changing sea levels—this book’s main theme? Plenty!

Tiny gas bubbles trapped in polar ice cores revealed that greenhouse gases also varied in sync with the ice age cycles. Carbon dioxide and methane played key roles in past climate changes, roles they still play at present and most likely will in the future, as well. At least eight times during the last million years, vast ice sheets blanketed much of the Northern Hemisphere and subsequently retreated. Sea level and greenhouse gas concentrations both rose and fell in tune with the ice sheets.

Climate skeptics point to such past climate and sea level swings as proof that we are merely experiencing another natural variation. However, anthropogenic atmospheric greenhouse gases are climbing. Carbon dioxide (394 parts per million in 2012) approaches levels last experienced in the balmier Pliocene epoch, around 3 million years ago, when sea levels reached over 66 feet (20 meters) above present. Temperatures now stand 1.0EF (0.6EC) above the mid-20th century average, with the nine warmest years in the 132-year instrumental record occurring since 2000.

The pace of sea level rise has quickened since the late 19th century, averaging around 1.7-1.8 mm/yr over most of the 20th century, going to 3 mm/yr since 1993, closely paralleling the rising trajectory of carbon dioxide and global temperature. Sea level is likely to rise even higher as the Earth’s climate warms, potentially threatening the coasts with more frequent and severe flooding, increased beach erosion, and saltwater encroachment into coastal lagoons, streams, and aquifers.

Rising Seas: Past Present, Future addresses the causes and consequences of sea level rise. Recent climate and sea level trends reinforce the book’s conclusions that we are moving toward an increasingly warmer and, quite likely, a more aquatic planet. We would do well to take heed and begin preparations to stem the oncoming tide.